Much evidence points to the idea that a meteorite gouged the Earth around 65 million years ago, triggering volcanic and climate changes that eventually wiped out the dinosaurs.

When the huge, mostly underwater crater was found off the Yucatan peninsula, it seemed the perfect candidate.

"Since the early 1990s the Chicxulub crater on Yucatan, Mexico, has been hailed as the smoking gun that proves the hypothesis that an asteroid killed the dinosaurs and caused the mass extinction of many other organisms at the Cretaceous-Tertiary (K-T) boundary 65 million years ago," the researchers wrote.

The K-T boundary can be seen in rock formations as a thin layer of clay rich in iridium, an element common in meteorites.

But the researchers said a core drilled out of the middle of the crater suggested it dated back more than 300,000 years before the K-T boundary and "thus did not cause the end-Cretaceous mass extinction as commonly believed".

The researchers, led by U.S. researcher Professor Gerta Keller of Princeton University in New Jersey and including experts from Germany, Switzerland and Mexico, studied a sample that extended 1500 metres below the current surface, in the middle of the more than 200 kilometre-wide crater.

Other samples have included tiny pieces of glass-like rock that could have been melted during an asteroid impact, and which seem to date to the 65-million-year point, give or take a few hundred thousand years.

But their core sample showed fossils that suggest the crater was blasted out 300,000 years before the K-T boundary. Magnetic evidence also suggested it was older than previously believed.

Chain of disasters

This finding would support an alternative theory that the dinosaurs and other forms of life were wiped out in a series of disasters that changed the Earth's climate, Keller's team said.

They noted there are other craters dating to around this time. None is big enough to have caused world-altering changes by itself.

But the meteorites hit at the same time of a busy period of volcanic activity known as Deccan volcanism, as well as when greenhouse-type atmospheric warming and major extinctions occurred.

The name Deccan comes from an area of what is now India where a massive amount of molten material surged up from near the Earth's core 65 million years ago.

It would have brought vast amounts of carbon gases to Earth's surface, causing a warming effect that would have wiped out many species of plants and animals.

"This finding suggests that the K-T boundary impact (and volcanism) may have been the straw that broke the camel's back, rather than the catastrophic kill of a healthy thriving community," the researchers concluded.

Now they need to find the actual crater left by whatever made this final blow.

Perhaps one known as the Shiva crater in India, dating to around the same period, is the one, they suggested.